A representative process of the process of producing an isomerized sugar syrup, comprising saccharifying starch to a saccharified solution and treating the saccharified solution with glucose isomerase comprises hydrolyzing starch with an enzyme to dextrin and hydrolyzing then dextrin with a different enzyme to a glucose solution, namely saccharified solution. The isomerization reaction of glucose with glucose isomerase to fructose is an equilibrium reaction, where the ratio of glucose and fructose in the isomerized sugar syrup is generally about 58:42. So as to overcome poor sweetness, further, purified fructose may sometimes be added. In that case, the final ratio of glucose and fructose is generally about 45:55. From the standpoint of low production cost, the isomerized sugar syrup is widely used as a sweetener in soft drinks and other drinks. In USA, the isomerized sugar syrup is consumed up to 8,000,000 tons or more yearly.
Attention has been focused recently on rare sugars because rare sugars have various physiological effects. Therefore, research works have been done intensively. For wide industrial application, furthermore, efficient production of these rare sugars is essential.
One of rare sugars, namely D-psicose is produced at a yield of 20 to 25 from D-fructose by allowing D-ketohexose.3-epimerase (patent reference 1) to react with D-fructose. A certain report tells that in case that D-psicose.3-epimerase (non-patent reference 1) is used, still furthermore, D-psicose is produced at a yield of about 40% and that in case that boric acid is used in combination, D-psicose is produced at about 60%. In case of industrially producing rare sugars at amass scale using the enzymes, the safety profiles of the enzymes should be validated first; and then, problems regarding bacterial cell culture and purification as an enzyme source and individual production steps of absorbents and the like should be overcome one after another.